This disclosure relates generally to magnetoresistance sensors for position and orientation determination, and more particularly to position and orientation tracking systems that use magnetic fields to determine the position and orientation of an object.
Position and orientation tracking systems are used in various industries and applications to provide position and orientation information relating to objects. For example, position and orientation tracking systems may be useful in aviation applications, industrial applications, security applications, game applications, animation applications, motion sensing applications, and medical applications. The technologies used by these systems vary to include electromagnetic (EM), radio frequency (RF), optical (line-of-sight), and mechanical.
In medical applications, position and orientation tracking systems are used to provide an operator (e.g., a physician or other medical professional) with information to assist in the precise and rapid positioning of a medical device located in or near a patient's body. In general, an image may be displayed on a monitor to provide positioning information to the operator. The image may include a visualization of the patient's anatomy with a graphical representation of the medical device displayed on the image. As the medical device is positioned with respect to the patient's body, the displayed image is updated to reflect the correct device coordinates. The base image of the patient's anatomy may be generated either prior to or during the medical procedure. For example, any suitable medical imaging technique, such as X-ray, computed tomography (CT), magnetic resonance (MR), positron emission tomography (PET), ultrasound, or any other suitable imaging technique, as well as any combinations thereof may be utilized to provide the base image displayed during tracking. After registering the base image to the position and orientation of the patient, or to the position and orientation of an anatomical feature or region of interest, the combination of the base image and the graphical representation of the tracked medical device provides position and orientation information that allows a medical practitioner to manipulate the device to a desired position and orientation.
To determine device location, position and orientation tracking systems may utilize EM sensors performing magnetic field generation and detection. At least one magnetic field is generated from one or more EM sensors (e.g., magnetic field generators or transmitters), and the at least one magnetic field is detected by one or more complementary EM sensors (e.g., magnetic field receivers). In such a system, the magnetic field may be detected by measuring the mutual inductance between the EM sensors. The measured values are processed to resolve a position and orientation of the EM sensors relative to one another.
EM sensors are typically implemented with coils to generate and detect the magnetic fields. While coil based EM sensors have been successfully implemented, they suffer from poor signal-to-noise ratio (SNR) as the transmitter coil frequency is reduced and/or the receiver coil volume is reduced. Reducing the SNR translates into a reduced range (distance from transmitter to receiver) of the EM sensors that may result in a clinically meaningful position error.
A problem associated with coil based EM sensors is that they are susceptible to magnetic field distortions that arise from eddy currents in nearby conducting objects. The tracking technique used with coil based EM sensors relies on a stable magnetic field, or a known magnetic field map. Therefore, unpredictable disturbances resulting from metallic objects in the magnetic field reduce the accuracy or may even render the tracking technique useless. Selecting a magnetic field frequency as low as the application allows reduces problems resulting from eddy currents, however it also reduces the sensitivity of coil based EM sensors since these are based on induction.
Other problems associated with coil based EM sensors is that they are generally expensive to manufacture and are also inherently sensitive to parasitic inductance and capacitance from the cables, connectors and electronics because the sensor signal is proportionally smaller while the parasitic signal remains the same. While some of the parasitic contributions may be partially nulled out using more expensive components and manufacturing processes, the remaining parasitic inductance and capacitance result in a reduced range.
In addition to coil based EM sensors, there are a large variety of magnetic sensors with differing price and performance attributes. Hall effect-sensors are typically used to detect fields down to approximately 10−6 Tesla. These sensors are stable, compact, relatively inexpensive and have a large dynamic range. Anisotropic magnetoresistive (AMR) sensors can detect fields down to approximately 10−9 Tesla. While these sensors are compact and relatively inexpensive, they are highly prone to drift and have a small dynamic range. Therefore AMR sensors need to be reinitialized frequently using high current pulses. Fluxgate magnetometers can detect fields down to approximately 10−11 Tesla. However these sensors are expensive, bulky and have a relatively small dynamic range. SQUID magnetometers can detect fields down to approximately 10−15 Tesla. They are also expensive with significant operating costs since they require cryogens or a high-power closed-cycle cooling system.
Therefore, there is a need for a position and orientation tracking system having magnetoresistance sensors that have a small form factor, excellent signal-to-noise ratio, excellent low frequency operation, lower sensitivity to parasitic inductance and capacitance, lower sensitivity to distortion, and are very low cost to manufacture.